Alkali vapor cells have been used extensively since the 1960s in the study of light-atom interactions. Vapor-cell applications, both proposed and realized, include atomic clocks, communication system switches and buffers, single-photon generators and detectors, gas-phase sensors, nonlinear frequency generators, and precision spectroscopy instrumentation. However, most of these applications, and cold-atom systems in general, have only been created in laboratory settings.
Macroscale vapor cells are widely used in macroscale atomic clocks and as spectroscopy references. Macroscale vapor cells are typically 10-100 cm3 in volume, which is insignificant for m3 scale atomic clocks, but far too large for chip-scale atomic clocks which are at most a few cm3 in volume.
A key driver has thus been to reduce vapor-cell size. Traditional vapor-cell systems are large and, if they have thermal control, have many discrete components and consume a large amount of power. To realize the full potential of vapor-cell technologies, the vapor-cell systems need to be miniaturized. Chip-scale atomic clocks and navigation systems require miniature vapor cells, typically containing cesium or rubidium, with narrow absorption peaks that are stable over time.
The amount of alkali vapor in a vapor cell changes over time as the vapor adsorbs, diffuses, and reacts with the walls. Alkali metal vapor pressure may be changed with a small set of known technologies (see Monroe et al., Phys Rev Lett 1990, 65, 1571; Scherer et al., J Vac Sci & Tech A 2012, 30; and Dugrain, Review of Scientific Instruments vol. 85, no. 8, p. 083112, August 2014). However, these systems are slow, complex, and/or have a short longevity.
Traditionally, alkali metals have been introduced into magneto-optical trap (MOT) vacuum systems via difficult-to-control preparation steps, such as manually crushing a sealed alkali-containing glass ampule inside a metal tube connected to the vacuum system via a control valve. See Wieman, American Journal of Physics vol. 63, no. 4, p. 317, 1995. This approach requires external heating to replenish the alkali metal inside the vacuum system as needed, which is a slow process with little control over the amount of alkali metal delivered. The manual labor is non-ideal for automated systems or chip-scale devices.
An alternative exists in the now-common alkali metal dispensers, which are effectively an oven-controlled source of alkali metal, whereby the desired alkali metal is released by chemical reaction when a current is passed through the device. While this process automates the release of alkali metal into the vacuum system, it has difficulty in fabrication compatibility with chip-scale cold-atom devices. Further, the timescales required for generating (warm up) and sinking (pump down) alkali are typically on the order of seconds to minutes, and can vary greatly depending on the amount of alkali metal built up on the vacuum system walls.
Double MOTs wherein the first MOT is loaded at moderate vacuum, and then an atom cloud is transferred to a second high-vacuum MOT, have been implemented on the laboratory scale. Again, these systems require complicated dual-vacuum systems and controls as well as a transfer system to move the atom cloud from one MOT to the other, none of which is amenable to chip-scale integration.
Light-induced atomic desorption is a recent technique that solves some of the long pump-down times by only releasing a small amount of alkali using a desorption laser; however, this method requires preparing a special desorption target in the MOT vacuum chamber. The desorption laser can interfere with the trapping lasers of the MOT (see Anderson et al., Physical Review A vol. 63, no. 2, January 2001). It also has yet to demonstrate suitable time constants below 1 second.
Thermoelectric stages can be used to regulate the overall temperature of the vapor cell, but this requires the addition of the thermoelectric stages, a temperature sensor and controller, and a significant amount of power (watts) to maintain the entire temperature of the cell at the correct temperature for MOT operation. The effectiveness of this approach will also depend on the overall size of the MOT cell and the efficiency of the thermoelectric stages, limiting the time constants at which the MOT can be loaded and the residual pressure stabilized.
An atom chip is an arrangement of microfabricated current-carrying wires patterned on a substrate which is used to trap and control atoms via the strong magnetic field gradients offered at distances close to conductors. Atoms chips enable highly sophisticated experiments to be condensed into areas on the order of a few square centimeters and readily lend themselves to the miniaturization and integration of cold atom systems for practical applications beyond the laboratory.
Atom chips use metal traces patterned via lithographic techniques to create magnetic fields involved in trapping populations of atoms. See U.S. Pat. No. 7,126,112 for “Cold atom system with atom chip wall”; Fortagh et al., Rev. Mod. Phys. 79, 235 (2007) Reichel et al., Atom Chips, Wiley, 2011; and Treutlein, Coherent manipulation of ultracold atoms on atom chips, Dissertation, Ludwig-Maximilians-University Munich, 2008, which are hereby incorporated by reference. Atom chips typically are implemented as one wall of a vapor cell. Thus they suffer from the same issues—such as slow vapor pressure rate of change and loss of alkali vapor to the walls—as conventional vapor cells. Improvements to conventional vapor cells in which magnetic trapping fields are generated by magnets or electromagnets outside the vapor cell also apply to atom chips for which magnetic fields are generated by magnets or electromagnets inside the vapor cell.
In uncoated atomic vapor cells under vacuum, orders of magnitude more atoms are adsorbed on solid surfaces than are present in the vapor phase. Also, interactions with vapor cell walls can limit the coherence time (or even the lifetime for unstable elements) of the atoms in the vapor.
Coatings have been applied to vapor cells in order to reduce the number of adsorbed atoms. Typical vapor cell coatings are long-chain aliphatic hydrocarbons (e.g. paraffin or octadecyltrichlorosilane). See, for example, Bouchiat and Brossel, “Relaxation of Optically Pumped Rb Atoms on Paraffin-Coated Walls”, Physical Review 147, 41, 1996, and Yi et al., “Method for characterizing self-assembled monolayers as antirelaxation wall coatings for alkali vapor cells”, Journal of Applied Physics 104, 023534, 2008, each of which is incorporated by reference. The processes for applying these coatings are tricky and typically leave a fraction of the wall uncoated, which limits their effectiveness in reducing the number of atoms adsorbed to cell walls. In the case of paraffin coatings, the temperature typically must remain below 50° C. As a consequence, the use of paraffin coatings in the case of alkali atoms is limited to Cs, Rb, and to a lesser extent K, which have sufficient vapor pressures for spectroscopic measurements at room temperature. See Lucchesini et al., “Low Energy Atomic Photodesorption from Organic Coatings”, Coatings 2016, 6, 47, which is incorporated by reference.
Alumina and borosilicate glass have been known as inorganic vapor-cell surfaces that are effective for low residence times. See Stephens et al., “Study of wall coatings for vapor-cell laser traps”, Journal of Applied Physics 76, 3479, 1994, which is incorporated by reference.
Although the need for coatings having low surface residence times for adsorbed vapor atoms has been known since the mid-1990s, no inorganic coatings better than alumina have been found to date. It is therefore desirable to provide superior, low-adsorption-energy coating materials for vapor cells.